Resting locations and upstream swimming pathways of Galaxias maculatus in response to spoiler baffle hydrodynamics

Increased concern over culverts’ capacity to enable fish passage has resulted in significant numbers of retrofitting solutions. The primary purpose of these solutions, such as the addition of spoiler baffles, is to break a culvert’s uniform flow into more natural heterogeneous flows with a series of high and low-velocity zones (HVZs and LVZs). This allows fish to pass when encountering HVZs in burst mode and to use LVZs as resting locations. Although quantitative assessments of various retrofitting solutions have shown improved culvert passage, a detailed understanding of the hydrodynamic requirements within these zones is not well established, especially for Southern Hemisphere small-bodied fish. In this study, a behavioural and hydrodynamic assessment of HVZs and LVZs was undertaken for Galaxias maculatus, a widespread small-bodied freshwater species of the Southern Hemisphere. Fish were studied in a laboratory flume whilst passing an area covered with spoiler baffles. Resting locations and upstream swimming pathways were delineated. Hydrodynamic characteristics for the identified regions of interest allowed for a comparative assessment of the studied spoiler baffle arrangements. We show that G. maculatus upstream swimming pathways predominantly cover areas with water velocity below 0.3 m/s, and turbulent kinetic energy (TKE) below 0.015 m2/s2. Horizontal Reynolds stresses and hydraulic drag are also calculated to characterise relevant areas. Identified resting locations have water velocities of less than 0.2 m/s and TKE below 0.012 m2/s2. Finally, a comparative assessment of spoiler baffle arrangements shows that a medium density patch generates more areas of desired HVZs and LVZs than the other tested patches. It is, therefore, recommended to test an arrangement of medium density spoiler baffles as a viable passage solution at a culvert’s full-length scale and varying slope.